1. Field of the Invention
The present invention relates to a motor drive circuit.
2. Description of the Related Art
In a motor drive circuit that controls the rotation of a motor, a motor coil is intermittently driven using a pulse signal so that the rotational speed of the motor may be adjusted. For example, there is a case where a pulse signal having a duty ratio according to a level of a control voltage is obtained by comparing a speed control voltage of a direct current input from a microcomputer, etc., with an oscillation voltage that varies in a triangle wave shape, and a motor coil is intermittently driven using the generated pulse signal (see, e.g., Japanese Patent Application Laid-open Publication No. 2001-320890).
Here, since the variation range of the speed control voltage may vary with specifications of the microcomputer, etc., there is a case where the variation range of the oscillation voltage can be set according to the variation range of the speed control voltage. In general, the oscillation voltage is generated by the following operation in a repetitive manner: charging a capacitor with a constant current; and thereafter starting discharging the capacitor with a constant current when the charging voltage of the capacitor reaches an upper limit level; and thereafter starting charging the capacitor with a constant current when the charging voltage of the capacitor reaches a lower limit level. For example, as illustrated in FIG. 6, when setting the upper limit level at V1 and the lower limit level at V2, an oscillation voltage Vosc1 is obtained which is indicated by a broken line; and when setting the upper limit level at V1 and the lower limit level at V3, an oscillation voltage Vosc2 is obtained which is indicated by a solid line. In other words, when setting the variation range of the speed control voltage Vd in the range of V1 to V2, the oscillation voltage Vosc1 is generated; and when setting the variation range of the speed control voltage Vd is set in the range of V1 to V3, the oscillation voltage Vosc2 is generated.
However, in a case where the variation range of the oscillation voltage, i.e., the amplitude of the oscillation voltage, is changed, assuming that the rate of change in oscillation voltage during charge/discharge is the same regardless of amplitude, the frequency of the oscillation voltage changes according to the amplitude. In an example of FIG. 6, assuming that V2=(V1+V3)/2, the frequency of the oscillation voltage Vosc1 is twice the frequency of the oscillation voltage Vosc2. Accordingly, the frequency of a pulse signal PWM1 obtained by comparing the oscillation voltage Vosc1 with the speed control voltage Vd is twice the frequency of a pulse signal PWM2 obtained by comparing the oscillation voltage Vosc2 with the speed control voltage Vd. Under circumstances where the duty ratio of the pulse signal thus changes according to the amplitude of the oscillation voltage, noise may occur when the motor is intermittently driven and the pulse signal may have a frequency incapable of driving the motor, due to frequencies of the pulse signal entering an audible region.